Here's a place where I can post my thoughts on new papers, provide updates on my projects, and post info that will eventually be on my website The Theropod Database - http://theropoddatabase.com/ . It will center on theropods, but may delve into other topics as well such as phylogenetics.

Thursday, April 28, 2011

Maxillary fenestra - a preview

Last time I described my in progress paper. While one of the downsides to preparing something for publication is that I have to keep the results to myself until it's out, here's a preview of what the paper will be like, using the classic "maxillary fenestra" character. So far I've only dealt with the characters and taxa of the first TWG matrix- Norell et al., 2001, NEA01 below). I list the improved version of the character, the old version, include comments on its original use and other issues, then list the distribution with explicit references (to two specimens if possible). Note while many of the codings can be backed up by personal observation, the literature references allow other workers to check even without access to the specimens. Entries with asterisks are those differing from Norell et al.'s codings, which are elaborated upon and figured when a coding is changed to another (unless the original coding was "?"). This makes coding differences explicit and transparent, and will facilitate resolution.

Comments- An extremely commonly used character in theropod analyses, Gauthier first (1984) used it to distinguish tetanurines then included it in a matrix for that purpose in 1986 (character 37).Note the medial wall of the maxillary antrum may not be fenestrated, leading to the appearance of a maxillary fossa instead of a fenestra as in Byronosaurus (IGM 100/983; Makovicky et al., 2003- fig. 7).The condition in oviraptorids is interpreted here differently than in most sources.The basal oviraptorosaur Incisivosaurus shows a large fenestra at the anterior rim of the antorbital fossa and a smaller fenestra posterodorsal to it.As these match the position of the promaxillary and maxillary fenestrae (respectively) in dromaeosaurids, they are homologized here.Oviraptorids have the same arrangement of fenestrae, but their promaxillary fenestra is enlarged and maxillary fenestra highly reduced, as exemplified by Citipati.

0- *Pelecanimimus LH 7777 (Perez-Moreno et al., 1994- fig. 2).Only one accessory fenestra is reported, which has usually been identified as a maxillary fenestra.However, its low position at the ventral border of the antorbital fossa matches ornithomimosaur promaxillary fenestrae and it is here identified as such.

Erlikosaurus IGM 100/111 (Clark et al., 1994- figs. 1 and 2). Note that the antorbital fossa wall is incomplete, but any fenestra would have to be positioned more ventrally than in other taxa if it was in the now broken area.

Chirostenotes ROM 43250 (Sues, 1997- fig. 1).While there are two fossae in the antorbital fossa, both are anterior to the maxillary antrum, which has a solid lateral wall.

*Gallimimus ZPAL MgD-I/1 (Osmolska et al., 1972- pl. 30).While Norell et al. (2001) coded this differently, the plate seems to show promaxillary and maxillary fenestrae separated by a horizontal bar as in other ornithomimosaurs.Kobayashi (2004) codes this as polymorphic in his version of the TWG matrix, perhaps indicating some specimens differ from ZPAL MgD-I/1.

*Confuciusornis GMV-2130 (Chiappe et al., 1999- fig. 15), GMV-2131 (Chiappe et al., 1999- fig. 16).While Norell et al. (2001) scored this feature as absent, the single accessory fenestra is here identified as a maxillary fenestra due to its posterior placement and the lack of a dorsal maxillary process which could contain a promaxillary recess.

?- Shuvuuia.Both Chiappe et al. (2002- 92, fig. 4.3) and Sereno (2001- fig. 12) identified maxillary fenestrae in IGM 100/977.However, the right maxillary fenestra in Chiappe et al.'s figure 4.3 is located far anterior to the left, and Sereno's figure 10 shows it is a break in the maxilla anterior to the antorbital fenestra.Sereno also places the maxillary fenestra anteriorly, and his reconstruction differs from the material in having a defined dorsal edge to an antorbital fossa anterior to the antorbital fenestra.While there at first glance does appear to be a well defined ventral edge, this is more probably caused by dorsoventral crushing which ventromedially displaced the portion of the maxilla anterior to the fenestra.This is confirmed by IGM 100/1001 (Chiappe et al., 2002- fig. 4.5), which shows no fossa anterior to the antorbital fenestra.The supposed left maxillary fenestra of Chiappe et al. is defined posteriorly by an anteriorly curved structure with a flared base, which does not contact the maxilla dorsally.This is congruent with a palatine, but not part of the maxilla.As Chiappe et al. (2002- 92) note, no specimen preserves a medial antorbital fossa wall, so the presence of a maxillary fenestra is unknown.

*Oviraptor (coded 1 by Norell et al., 2001).Norell et al. coded this based on what is here identified as the promaxillary fenestra.While there is a hole in the proper position for a maxillary fenestra in the left maxilla (AMNH 6517, Clark et al., 2002- fig. 11), numerous other holes are also present which are caused by damage.

"Ingenia" (coded 0 by Lu, 2004).As Lu identifies the promaxillary fenestra of oviraptorids as a maxillary fenestra, his statement "Ingenia" lacks the opening is here applied to the promaxillary fenestra.

2 comments:

Or some taxa lose the promaxillary or maxillary fenestra. A third fenestra may be formed -- or irregularly positioned it might be more likely -- to associate with the apparent identification by some authors of fenestra by space between antorbital fenestra and the rostral limit of the fossa. Such accessory fenestra, such as the highly dorsal "maxillary fenestra" in Incisivosaurus, or the rostral maxillary "foramina" in virtually all oviraptorids, may not represent homologues to other taxa (dromaeosaurs, in this case). It's shocking, I know, but oviraptorids and some of their antecedents have come up with a lot of external vascularization and pneumatization that may object to being "true" fenestrae, and thus useful to code in the matrix.

It's true any given feature may not be homologous to similar features in other taxa, but we can only find out via phylogenetic analysis. In Incisivosaurus' case, what I identify as the promaxillary fenestra does connect to the promaxillary recess in the anterior maxillary ramus (CT scan in fig. 9E of Balanoff et al., 2009). So this, along with the presence of a more posteriorly positioned fenestra, supports its homology. Maybe I'm wrong, but my interpretation certainly seems more parsimonious than losing the promaxillary fenestra, gaining a posterodorsal accessory fenestra, and having the connection with the promaxillary recess be a fenestra communicans (sensu Witmer, 1997; variably present in theropods) instead.